Virologic failure occurs in a large proportion of individuals receiving treatment with combination antiretroviral therapy. Studies suggest that treatment interruption prior to initiation of a multiple-drug rescue regimen may improve virologic response in individuals who have failed several prior antiretroviral regimens. Other studies suggest there is a virologic benefit derived from using genotypic or phenotypic resistance testing in selecting salvage therapy regimens for patients failing antiretroviral therapy. This study tests the hypothesis that salvage regimens selected on the basis of HIV-1 resistance genotype, phenotype [AS PER AMENDMENT 02/19/02: virtual phenotype], and treatment history will be more effective if there is a period of treatment interruption before initiating that regimen.